There is currently a demand for high speed data transmission cables which are capable of high-fidelity signal transmission at minimal signal attenuation. The ever-increasing use of high speed computer equipment and telecommunications equipment has increased such demand.
One existing cable product capable of high data rate transmission is fiber-optic cable which has good bandwidth performance over long distances. Furthermore, fiber-optic cables provide very low attenuation and little interference or noise with the transmitted signal. However, despite their desirable signal transmission qualities, fiber-optic cables are still very expensive. Furthermore, when transmission of signals over shorter distances is required, fiber-optic cables become even less desirable from an economic standpoint. For high speed data transmission over relatively short distances, such as up to 50 meters, copper based, differential signal transmission cables are the predominant choice in the industry.
Differential signal transmission involves the use of a cable having a parallel pair of conductors wherein the information or data which is transmitted is represented by a difference in voltage between the parallel conductors. The data is represented in transmission by polarity reversals on the conductor pair and the receiver or other equipment coupled to the receiving end of the cable determines the relative voltage difference between the conductors and the difference is analyzed to determine its logical value, such as a 0 or 1. Differential pairs may be shielded or unshielded. Shielded differential pairs generally perform better than unshielded pairs because the internal and external environments of the conductors are isolated. Improved attenuation performance also usually results with shielded cables.
Differential signal transmission cables have a variety of desirable electrical characteristics, including immunity to electrical noise or other electrical interferences. Since the differential signals transmitted are 180.degree. out of phase to provide a balanced signal in the cable, and are considered to be complementary to one another, any noise will affect both of the conductors equally. Therefore, the differences in the signals between the conductor pairs due to external electrical noise and interference are generally negated, particularly for shielded pairs. It may also be true for unshielded differential pairs as well by varying the twisting of the pairs, for example. Differential signal transmission cables are also immune to cross-talk, that is, interference between the individual cables due to the signals on other cables which are bundled together into a multi-cable structure. Again, shielded differential pairs will generally outperform unshielded pairs with respect to cross-talk. Multiple differential signal cables in a larger overall cable structure are referred to as primary cables.
Since differential signal transmission relies upon parallel transmission of the data signal and comparison of the differences in those signals at the receiving end of the cable, it is desired that the corresponding signals of each pair arrive at the receiving end at the same time. Because of insulative property differences experienced by each conductor of a cable pair, such as differences due to dielectric inconsistencies and/or physical characteristics of the cable, differential signal transmission cables are subject to signal skew. Signal skew is defined as the delay of the arrival of one of the corresponding or complimentary signals at the receiving end with respect to the other signal. In simpler terms, one complimentary signal arrives at the receiving end faster than the other signal, a condition which is exaggerated as cable length increases. Generally, a signal skew budget is designed into data transmission systems and the cables which link the systems are allowed only a portion of the budget.
Therefore, signal skew is one of the important parameters which must be considered when using a differential signal transmission cable. As will be appreciated, it is desirable to keep signal skew in a cable to a minimum to prevent errors in communication. Furthermore, low signal skew is necessary for proper cancellation of noise, because if the two opposing signals do not arrive at the receiving end at the same time, a certain amount of the noise in the cable will not be cancelled. A lower signal skew will also minimize jitter, the amount of real time it takes for the signal rising and falling edges to cross, which allows a differential signal transmission cable to be utilized at greater lengths or distances. It is therefore desirable to utilize a data transmission cable having a relatively low signal skew.
Another desirable characteristic in differential signal data transmission cables is low attenuation. Attenuation will generally be affected by the physical structure of the cable, which includes the shield type and design, the dielectric material type, and the conductor type, the position of the conductors, and the electrical interaction between the conductors of a cable. If the primaries are poorly constructed, the dielectric material properties, conductor-to-dielectric geometry, and hence impedance characteristics, may vary along its length, thus increasing its signal attenuation or loss characteristics when the cable is subjected to use. Accordingly, it is desirable to utilize a cable which has low attenuation characteristics at the desired operating frequency, so that cable length can be maximized, and also a cable which maintains a constant, low attenuation characteristic during use.
To that end, it is further desirable to maintain the conductors in consistent positions within the insulation and with respect to one another. It is also desirable to maintain consistent dielectric properties of the cable insulation along its length to reduce impedance variations and hence reduce attenuation and signal skew. At the same time, high speed data transmission cables should be flexible and able to withstand the mechanical abuses associated with usage. For example, the distance between the primaries, as well as the distance from the center of each primary to the outer surface of the dielectric, should be consistent along the length of the cable.
Accordingly, it is an objective of the present invention to provide a high-speed data transmission cable which produces relatively low signal skew, and minimizes signal attenuation within a high-speed data transmission cable at the particular driven frequencies of the cable.
It is still a further objective of the present invention to provide a high-speed data transmission cable which can be used at greater lengths than the present high speed data cables.
It is still a further objective of the invention to maintain the integrity of the data signal transmitted through the cable and to thus minimize the delay, distortion and attenuation of that signal.
It is another objective of the present invention to provide a flexible and durable high-speed data transmission cable which maintains a more consistent dielectric constant along its length.